“Hologram transmission through multi-mode optical fibers”, Optical Express 19, 247-254 (2011), R. D. Leonardo and S. Bianchi discloses one approach for concentrating light output from a multi-mode fiber (MMF) into one or several spots using a direct or blind search method. Such a method relies upon a phase only analysis of the optical field input to the MMF. This may, however, lead to loss of optical power and/or the generation of noise when creating one or several spots at the output of the MMF.
Nature Photonics, 4, 388-394 (2010), T. {hacek over (C)}i{hacek over (z)}már, M. Mazilu and K. Dholakia discloses a method of controlling the transmission of light within a highly scattering and diffusive, but essentially polarization preserving medium and, in particular, within biological tissue. The method comprises characterizing an optical system including the biological tissue in terms of both amplitude and phase of an optical field detected using a single intensity probe. Subsequently a complex field transformation is used to control an input optical field according to the determined amplitudes and phases of the optical field so as to provide a desired intensity within the biological tissue. Such a method may lead to loss of optical power and/or the generation of noise when creating one or several spots within the medium. Such a method also relies upon the use of a reference path defined internally within the biological tissue. Using such an internal reference path may cause several problems when controlling the transmission of light within a medium. For example, the use of an internal reference path may lead to inaccuracies during the characterization of the optical system and contribute to a loss of optical power when creating one or more intensity spots. The use of an internal reference path may, in some circumstances, even lead to failure of the method altogether. A reference optical field generated as a consequence of transmission of light along an internal reference path may exhibit a speckled character with an exponential distribution of intensity such that some parts of the reference optical field have very high intensities whilst others may have extremely low intensities, so termed ‘blind spots’, which cannot be used in the characterization of the optical system. An intensity detector having a very large dynamic range and high resolution may then be required to detect the intensity of the reference optical field and/or of any optical interference field generated between light transmitted along the internal reference path and light transmitted along a measurement path extending through the medium.